Aggregation and Reactivity of Organozincate Anions Probed by

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Organometallics 2009, 28, 100–110

Aggregation and Reactivity of Organozincate Anions Probed by Electrospray Mass Spectrometry Konrad Koszinowski* and Petra Bo¨hrer Department Chemie und Biochemie, Ludwig-Maximilians-UniVersita¨t Mu¨nchen, Butenandtstr. 5-13, 81377 Mu¨nchen, Germany ReceiVed July 23, 2008

Electrospray ionization (ESI) of mixtures of organolithium compounds and zinc chloride in tetrahydrofuran produced manifold mono- and polynuclear organozincate anions. Formation of the latter is strongly favored by the incorporation of chloride ligands, which apparently adopt bridging binding modes. Analysis of LinBu/ZnCl2 solutions at different concentrations showed that the relative ESI signal intensities for anions in different aggregation states closely correlate with their expected equilibria in solution. Moreover, the uni- and bimolecular gas-phase reactivity of the mass-selected anionic organozincates was studied. Upon collision-induced dissociation, most of these complexes lose a neutral metal fragment, and only the tributylzincates ZnBu3- react by elimination of alkenes. The tributylzincate complexes were also found to undergo ion-molecule reactions with formic acid. The relative rates for these proton-transfer processes decrease in the series ZnnBu3-, ZnsBu3-, and ZntBu3-, and they also decrease if the butyl groups are substituted for chloride ligands. These trends fully agree with the known solution-phase chemistry of organozinc compounds. 1. Introduction Lithium organozincate complexes LiZnR3 and Li2ZnR4 (R ) organyl)1 form in the reactions of ZnCl2 and ZnR2 with LiR (Scheme 1). These organozincate complexes show a higher nucleophilic reactivity than diorganyl zinc compounds ZnR2,2,3 while still preserving the high chemoselectivity characteristic of organozinc compounds.4,5 Because of this combination of favorable properties, organozincates are valuable reagents in organic synthesis. As such, their applications range from halogen-zinc exchange,6-9 carbozincation,8 and Michael-type reactions8,10 to the initiation of anionic polymerizations11 and epoxide ring openings.8 Despite this practical importance, many facets of organozincate chemistry are only poorly understood. First of all, the formulas LiZnR3 and Li2ZnR4 refer to the overall composition of the reactive mixtures but do not necessarily reflect the stoichiometry of the actual species present in solution. Hein and Schramm performed cryoscopic measurements on solutions of diethylzinc and ethyllithium in benzene and observed the formation of aggregates with high molecular weights.12 Toppet, * To whom correspondence should be addressed. E-Mail: Konrad. [email protected]. (1) Linton, D. J.; Schooler, P.; Wheatley, A. E. H. Coord. Chem. ReV. 2001, 223, 53. (2) Maclin, K. M.; Richey, H. G., Jr. J. Org. Chem. 2002, 67, 4602. (3) Uchiyama, M.; Nakamura, S.; Ohwada, T.; Nakamura, M.; Nakamura, E. J. Am. Chem. Soc. 2004, 126, 10897. (4) Knochel, P.; Singer, R. D. Chem. ReV. 1993, 93, 2117. (5) Knochel, P.; Perea, J. J. A.; Jones, P. Tetrahedron 1998, 54, 8275. (6) Harada, T.; Katsuhira, T.; Hattori, K.; Oku, A. J. Org. Chem. 1993, 58, 2958. (7) Kondo, Y.; Fujinami, M.; Uchiyama, M.; Sakamoto, T. J. Chem. Soc., Perkin Trans. 1 1997, 799. (8) Uchiyama, M.; Kameda, M.; Mishima, O.; Yokoyama, N.; Koike, M.; Kondo, Y.; Sakamoto, T. J. Am. Chem. Soc. 1998, 120, 4934. (9) Uchiyama, M.; Furuyama, T.; Kobayashi, M.; Matsumoto, Y.; Tanaka, K. J. Am. Chem. Soc. 2006, 128, 8404. (10) Isobe, M.; Kondo, S.; Nagasawa, N.; Goto, T. Chem. Lett. 1977, 679.

Scheme 1

Slinckx, and Smets could establish by NMR spectroscopy that the addition of tetrahydrofuran (thf) to such solutions resulted in deaggregation.13 NMR-spectroscopic measurements by Seitz and co-workers revealed that mixtures of dimethylzinc and methyllithium in thf contain complexes of LiZn(CH3)3 and Li2Zn(CH3)4 stoichiometries,14 whereas only Li2Zn(CH3)4 and Li3Zn(CH3)5 appear to exist in diethyl ether at low temperatures.15 More recently, Uchiyama and co-workers used a combination of spectroscopic techniques to characterize the zincate complexes present in thf solutions of Li2Zn(CH3)4, Li2Zn(CH3)3(CN), and Li2Zn(CH3)3(SCN).8 Their results point to the presence of tetracoordinate organozincate species.8 In addition to controlling the aggregation of organozincates, the solvent presumably also has a key influence on their dissociation equilibria. For the related lithium organocuprate complexes, solvent-separated ion pairs are known to prevail in thf.16 In contrast, contact ion pairs predominate in diethylether, which has poorer coordinating capabilities.16 A similar situation may (11) Kobayashi, M.; Matsumoto, Y.; Uchiyama, M.; Ohwada, T. Macromolecules 2004, 37, 4339. (12) Hein, F.; Schramm, H. Z. Phys. Chem. 1930, A151, 234. (13) Toppet, S.; Slinckx, G.; Smets, G. J. Organomet. Chem. 1967, 9, 205. (14) Seitz, L. M.; Little, B. F. J. Organomet. Chem. 1969, 18, 227. (15) Seitz, L. M.; Brown, T. L. J. Am. Chem. Soc. 1966, 88, 4140. (16) John, M.; Auel, C.; Behrens, C.; Marsch, M.; Harms, K.; Bosold, F.; Gschwind, R. M.; Rajamohanan, P. R.; Boche, G. Chem. Eur. J. 2000, 6, 3060.

10.1021/om8007037 CCC: $40.75  2009 American Chemical Society Publication on Web 12/15/2008

Aggregation and ReactiVity of Organozincate Anions

be expected in the case of lithium organozincates although only few examples of solvent-separated ion pairs have been reported for these.17 In order to obtain further insight into the nature of organozincate complexes in thf solution, we employed electrosprayionization (ESI) mass spectrometry. In contrast to NMR spectroscopy and cryoscopy, ESI mass spectrometry selectively probes the charged components present in solution and thus seems particularly well-suited for the detection of solventseparated organozincate complexes. Moreover, mass spectrometry provides a straightforward means to establish the stoichiometry of ionic species (for potential complications due to fragmentation during the ionization process, see below). Thanks to these assets, ESI mass spectrometry has recently become one of the chief methods to characterize ionic species in organometallic chemistry.18,19 While so far, most studies have focused on cations, Lipshutz and co-workers already in 1999 successfully used ESI mass spectrometry for the identification of organometallate complexes, namely organocuprates.20 Besides taking advantage of its analytical power, we also used mass spectrometry to characterize the reactivity of ionic species. Gas-phase experiments on mass-selected ions have substantially contributed to our current understanding of organometallic reactivity.19-29 Such experiments neglect solvent effects but have the great advantage of excluding complex equilibria, which can severely complicate reactivity studies in solution. With respect to organometallate complexes, O’Hair and co-workers have extensively investigated the uni- and bimolecular gas-phase reactivity of organomagnesates30 and their heavier homologues,31 as well as of organocuprates32,33 and -argentates,32 all prepared from gaseous precursors. We have performed similar gas-phase experiments on organozincate complexes extracted directly from solution and compare the obtained gasphase data with the trends known for the corresponding solution chemistry. Such a comparison promises to reveal to what extent the system’s intrinsic reactivity probed in the gas phase does correlate with its reactivity in solution and under which circumstances gas-phase data can be used for predictions on solution chemistry. In the present study, we have investigated organozincates prepared by the transmetalation of ZnCl2 (as well as of ZnBr2 and ZnI2) with organolithium compounds LiR. We not only (17) Westerhausen, M.; Wieneke, M.; Ponikwar, W.; No¨th, H.; Schwarz, W. Organometallics 1998, 17, 1438. (18) Plattner, D. A. Int. J. Mass Spectrom. 2001, 207, 125. (19) Chen, P. Angew. Chem. 2003, 115, 2938; Angew. Chem. Int. Ed. 2003, 42, 2832. (20) Lipshutz, B. H.; Keith, J.; Buzard, D. J. Organometallics 1999, 18, 1571. (21) Armentrout, P. B.; Beauchamp, J. L. J. Am. Chem. Soc. 1981, 103, 784. (22) Squires, R. R. Chem. ReV. 1987, 87, 623. (23) Hanratty, M. A.; Beauchamp, J. L.; Illies, A. J.; van Koppen, P.; Bowers, M. T. J. Am. Chem. Soc. 1988, 110, 1. (24) Schwarz, H. Acc. Chem. Res. 1989, 22, 282. (25) Eller, K.; Schwarz, H. Chem. ReV. 1991, 91, 1121. (26) Freiser, B. S. Acc. Chem. Res. 1994, 27, 353. (27) Schro¨der, D.; Schwarz, H. Angew. Chem. 1995, 107, 2126; Angew. Chem., Int. Ed. Engl. 1995, 34, 1973. (28) Armentrout, P. B. Acc. Chem. Res. 1995, 28, 430. (29) Schro¨der, D.; Shaik, S.; Schwarz, H. Acc. Chem. Res. 2000, 33, 139. (30) O’Hair, R. A. J.; Vrkic, A. K.; James, P. F. J. Am. Chem. Soc. 2004, 126, 12173. (31) Jacob, A. P.; James, P. F.; O’Hair, R. A. J. Int. J. Mass Spectrom. 2006, 255/256, 45. (32) James, P. F.; O’Hair, R. A. J. Org. Lett. 2004, 6, 2761. (33) Rijs, N.; Khairallah, G. N.; Waters, T.; O’Hair, R. A. J. J. Am. Chem. Soc. 2008, 130, 1069.

Organometallics, Vol. 28, No. 1, 2009 101

consider homoleptic organozincate complexes, but also include mixed organochlorozincates, such as ZnRCl2- and ZnR2Cl-. These species are likely intermediates in the formation of homoleptic organozincates starting from ZnCl2 and LiR (Scheme 1). Among the different organozincates studied, we particularly focus on those bearing butyl groups. LiZntBu3 and Li2ZntBu4 are arguably the organozincates most commonly used for synthetic applications,7,9,11 and it will be interesting to see whether the complexes ZntBunCl3-n- differ in their reactivity from their counterparts ZnnBunCl3-n- and ZnsBunCl3-n- (1 e n e 3). To answer this question, we probed the bimolecular gas-phase reactivity of these species toward methyl iodide and formic acid as prototypical examples of carbon electrophiles and proton donors, respectively.

2. Experimental Section 2.1. Materials and General Synthetic Methods. Tetrahydrofuran was distilled from potassium/benzophenone. ZnCl2 was dried by repeatedly heating it under high vacuum until it melted. Solutions of organolithium compounds LiR in alkanes were used as purchased. Their exact concentrations were determined by titration with 1,3-diphenyl-2-propanone tosylhydrazone.34 Standard Schlenk techniques were applied in all cases. 2.2. Preparation of LiR/ZnCl2 and 2 LiR/ZnCl2 Solutions. To a solution of dry ZnCl2 in thf, 1 or 2 equiv., respectively, of the organolithium compound LiR were added under argon. The resulting solution (c ) 0.1 mol L-1) was stirred at room temperature for 24 h and then diluted to 1 e c e 50 mmol L-1 before analysis by ESI mass spectrometry. 2.3. Preparation of 4 LiR/ZnCl2 Solutions.9 To a solution of dry ZnCl2 in thf cooled down to -78 °C, 4 equiv. of the organolithium compound LiR were added under argon. The resulting solution (c ) 0.1 mol L-1) was allowed to warm up to 0 °C and was stirred at this temperature for 2 h. The solution was diluted to c ) 50 mmol before analysis by ESI mass spectrometry. 2.4. ESI Mass Spectrometry. Samples of the thf solutions of the lithium organozincate complexes were transferred into a gastight syringe and administered into the ESI source of a TSQ 7000 multistage mass spectrometer (Thermo-Finnigan) by means of a syringe pump (Harvard apparatus). Typical flow rates ranged from 5 to 20 µL min-1. Particular care was taken to exclude or minimize contact of the organometallic samples with air. Traces of moisture or oxygen in the inlet system were eliminated by extensively flushing it with dry thf before adding the organometallic sample. The sample solution entered the ESI source via a fused-silica tube (0.10 mm inner diameter). Stable electrospray conditions were usually achieved for ESI voltages ranging from 3.0 to 4.3 kV with nitrogen as a sheath gas (2.5 bar). The spray then passed through a heated capillary held at 60 °C. This relatively low temperature yielded acceptable signal intensities while preventing excessive fragmentation of the potentially labile organozincate complexes. Furthermore, the potential difference between the heated capillary and the following electrooptic lens was kept low to avoid strong acceleration of the ions and unwanted fragmentations due to energetic collisions with gas molecules present in the ESI source region. Similar ESI conditions were maintained for all experiments to ensure direct comparability. The m/z ratios of the ions were established by scanning the first quadrupole mass filter. The ions then passed an octopole and a second quadrupole mass filter before reaching the detector. In response to the comments of one of the reviewers, we repeated the ESI-mass spectrometric analysis of a solution of LitBu/ZnCl2. This experiment was not performed with the TSQ 7000 instrument (34) Lipton, M. F.; Sorensen, C. M.; Sadler, A. C.; Shapiro, R. H. J. Organomet. Chem. 1980, 186, 155.

102 Organometallics, Vol. 28, No. 1, 2009

Koszinowski and Bo¨hrer

but instead with a quadrupole ion trap mass spectrometer (LTQ, Finnigan). The ESI conditions in this additional experiment were similar to those applied in the rest of the work. 2.5. Gas-Phase Reactivity Studies. For probing the uni- or bimolecular gas-phase reactivity of an organozincate ion of a particular m/z ratio, the first quadrupole mass filter was used to mass-select this ion. The mass-selected ion then passed the octopole, to which volatile substrates could be added via a needle valve and a home-built inlet system. The inlet system could be evacuated in order to avoid contamination of the substrates with air. For studying the unimolecular reactivity of a mass-selected ion in a collisioninduced dissociation (CID), argon (Linde, 99.998% purity) was used as collision gas. For the bimolecular reactions of the organozincate ions with methyl iodide and formic acid, these substrates were purified by repeated freeze-pump-thaw cycles. The m/z ratios of the product ions formed by CID or ion-molecule reactions were then established by scanning the second quadrupole mass filter before the ions reached the detector. The vacuum chamber of the mass spectrometer was held at T ≈ 343 K, and we assume that this temperature also describes the distribution of the internal energy of the neutral reactants. The collision energy ELABORATORY could be controlled by changing the voltage offset of the octopole Voct. To determine the true zero point in ELABORATORY, we varied Voct while measuring the absolute signal intensity (so-called potential-retardation analysis). The derivative of the obtained curve with respect to Voct then yielded the kinetic energy distribution of the ions with a maximum at the true zero point in ELABORATORY. These energy distributions were Gaussian-shaped with typical full widths at half-maximum of 1.6 ( 0.3 eV. The zero point in ELABORATORY for the different measurements was constant within ( 0.3 eV. Note that these potential-retardation analyses were not performed for the organozincate ions themselves but for CH3O-, Cl-, Br-, PhO-, and Na(OPh)2-, because higher and more stable signal intensities could be achieved for these anions (by ESI of methanolic solutions of their sodium salts). As the instrumental parameters applied in the potential-retardation analyses were very similar to those used for the investigation of the organozincate ions, we assume that the latter had kinetic energy distributions comparable to those observed for CH3O-, Cl-, Br-, PhO-, and Na(OPh)2-. For CID experiments, typically three different values for Voct were applied, corresponding to low (1.5 eV), medium (∼12 eV), and high (∼20 eV) collision energies ELABORATORY. These energies cannot be easily converted to collision energies in the center-ofmass frame because multiple collisions are likely at the typical argon pressure applied (p(Ar) ≈ 0.6 mtorr in the 18 cm-long interaction region as measured with a Convectron). The ion-molecule reactions of organozincate ions with methyl iodide and formic acid were studied at a collision energy of ELABORATORY ) 0 eV and pressures in the range of 0.1 e p e 0.5 mtorr (uncorrected reading of the Convectron). The lower limit of this range roughly corresponds to single-collision conditions. Each reaction was studied at three or more different pressures under pseudofirst order conditions (excess of the neutral substrate). In addition, the well-known35 ion-molecule reactions of the anions and CH3O-, HS-, Cl-, and Br- with methyl iodide were considered for comparison. As expected, in all cases the formation of I- was observed and thus indicated the occurrence of a nucleophilic substitution, as demonstrated in eq 1 for Cl-.

Cl- + CH3I f I- + CH3Cl

(1)

The relative rate constants determined for the reactions of CH3O-, HS-, Cl-, and Br- with methyl iodide show the same trend as data obtained from a selected-ion flow-tube study performed at room temperature35 but are not in quantitative agreement with the latter (35) Gronert, S.; DePuy, C. H.; Bierbaum, V. M. J. Am. Chem. Soc. 1991, 113, 4009.

Figure 1. Anion-mode ESI mass spectrum of a 10-mmolar solution of LinBu/ZnCl2 in thf. (a) Overview. (b) Comparison of observed (black) and expected isotopic patterns (red) for the low mass range. (Table S1 of the Supporting Information). Possible reasons for this disagreement could be the deviation in temperature between the two experiments or the absence of an inert bath gas in the present study. In general, both the rather broad kinetic energy distribution of the ions and the errors inherent in the measurement of the pressure are likely to limit the accuracy of the relative rate constants that can be obtained with our commercial instrument.

3. Results 3.1. Solutions of LiR/ZnCl2 3.1.1. LinBu/ZnCl2. Anionmode ESI of a 10-mmolar solution of LinBu/ZnCl2 in thf yielded rich mass spectra (Figure 1a). Besides the purely inorganic zincate complex ZnCl3-, organozincate ions were observed at higher m/z ratios. A first indication of their identity is given by their exact m/z ratios. While m/z ) 168.8 observed for 64Zn35Cl3clearly reflects the large mass defects of zinc and chlorine, these are apparently compensated for in the case of the ion at m/z ) 191.0. This ion therefore must contain several hydrogen atoms and thus most likely bears a butyl group. Hence, the assignment as 64ZnnBu35Cl2- is straightforward. In addition to this mononuclear species, the polynuclear complexes LiZnCl4-, LiZnnBuCl3-, Zn2nBuCl4-, and LiZn2nBu2Cl4- were found in the m/z range probed. For all of these ions, the measured isotopic patterns fully agree with those calculated for the expected zincates based on the natural abundances of the different isotopes (see Figure 1b for the low mass range).36 The identities of the organozincates were further (36) Isotope patterns can be conveniently calculated with the help of web-based resources, such as: Yan, J. Isotope Pattern Calculator v4.0 (http:// www.geocities.com/junhuayan/pattern.htm).



Table 1. Fragmentation Reactions of Mass-Selected Organozincate Anions fragment iona

parent ion entry 1 2a b 3 4a b c d e 5 6 7a b 8a b 9 10 11 b c d 12 13 14 15 16 17 18a b 19a b 20a b c d e f g 21a b 22a b 23

assignmentb

m/z 191 235

64

327 393

64

193 191 351

64

333

64

149 287 309

64

219 213 213 213 129 265 235

64

235

64

267

64

235

64

111

64

313

64

n

35

-

Zn Bu Cl2 Li64ZnnBu35Cl237ClZn2nBu35Cl337ClLi64Zn2nBu235Cl237Cl2-

ZnsBu35Cl37ClZntBu35Cl264 Zn66ZntBu235Cl237Cl64

Zn66ZntBu2(OH)35Cl37Cl-

Zn(CH3)35Cl2Zn66Zn(CH3)35Cl337ClLi64Zn66Zn(CH3)235Cl337Cl-

64

Zn(C4H3S)35Cl37ClZnnBu235Cl64 ZnsBu235Cl64 ZntBu235Cl64 Zn(CH3)235Cl64 Zn(C4H3S)235Cl64 ZnnBu364

ZnsBu3s

s

-

Zn Bu2( BuOO)

ZntBu3Zn(CH3)2(OH)Zn(C4H3S)3-

assignmentb

m/z 35 77 193 171 37 77 171 215 235 35 35 173 193 155 195 35 173 79 171 193 215 37 35 35 35 35 35 179 123 179 123 33 67 71 73 89 123 211 179 123 17 95 83

a

35

-

Cl Li35Cl264 ZnnBu35Cl37Cl64 Zn35Cl237Cl37 Cl Li35Cl264 Zn35Cl237ClLi64Zn35Cl237Cl2Li64ZnnBu35Cl237Cl35 Cl 35 Cl 66 Zn35Cl237Cl66 ZntBu35Cl266 Zn(OH)35Cl37Cl66 ZntBu35Cl37Cl35 Cl 66 Zn35Cl237ClLi35Cl37Cl64 Zn35Cl237ClLi64Zn(CH3)35Cl237ClLi66Zn35Cl337Cl37 Cl 35 Cl 35 Cl 35 Cl 35 Cl 35 Cl 64 ZnHnBu264 ZnH2nBu64 ZnHsBu264 ZnH2sBuHOO64 ZnH3C4H7OC3H5O2s BuOO64 ZnsBuH2 64 ZnHsBu(OOsBu)64 ZnHtBu264 ZnH2tBuOH64 Zn(CH3)OC4H3S-

neutral fragment(s) ∆m 156 158 42 156 356 316 222 178 158 158 156 178 158 178 138 114 114 230 138 116 94 182 178 178 178 94 230 56 112 56 112 234 200 196 194 178 144 56 56 112 94 16 230

assignment 64

n

Zn Bu Cl ZnnBu37Cl Li35Cl 64 ZnnBu35Cl Li64Zn2nBu235Cl237Cl 64 Zn2nBu237Cl2 Li64ZnnBu237Cl 64 ZnnBu2 64 ZnnBu37Cl 64 ZnsBu37Cl 64 ZntBu35Cl 64 ZntBu2 64 ZntBu37Cl 64 ZntBu2 64 ZntBu(OH) 64 Zn(CH3)35Cl 64 Zn(CH3)35Cl 64 Zn66Zn(CH3)235Cl2 Li66Zn(CH3)235Cl 66 Zn(CH3)35Cl 64 Zn(CH3)2 64 Zn(C4H3S)35Cl 64 ZnnBu2 64 ZnsBu2 64 ZntBu2 64 Zn(CH3)2 64 Zn(C4H3S)2 C4H8 2 C4H8 C4H8 2 C4H8 64 ZnsBu2, C4H8 2 C4H8, C4H8O2 64 ZnsBu2, H2O 64 ZnsBu2, CH4 64 ZnsBu2 C4H8, C4H8O2 C4H8 C4H8 2 C4H8 64 Zn(CH3)2 CH4 64 Zn(C4H3S)2 64

For isotopic or isotopologue fragment ions, respectively, observed at neighboring m/z ratios only one major component is listed. isotopologue is given. In several cases, additional isotopologues will significantly contribute to the signal intensity of the observed ion.

confirmed by CID experiments, which gave the expected fragment ions: mononuclear ZnnBuCl2- produced Cl- as ionic fragment (Table 1, entry 1, and Figure S1 of the Supporting Information), and the polynuclear species mainly formed LiCl2and/or zincate ions (Table 1, entries 2-4, and Figures S2–S4 of the Supporting Information). The observed signal intensities exhibited a clear dependence on the concentration. At c(LinBu/ZnCl2) ) 1 mmol L-1, ZnCl3predominates, whereas at higher concentrations the polynuclear complexes Zn2nBuCl4-, and LiZn2nBu2Cl4- prevail (Figure 2). The predominance of the purely inorganic ZnCl3- at low concentrations possibly results from the partial decomposition of the organometallic species by residual traces of moisture and/ or oxygen. The effect of such contaminants should be most pronounced for low concentrations c(LinBu/ZnCl2). 3.1.2. LisBu/ZnCl2 and LitBu/ZnCl2. Anion-mode ESI of solutions of LisBu/ZnCl2 and LitBu/ZnCl2 in thf gave results similar to those obtained for LinBu/ZnCl2. Besides ZnCl3-, the direct analogues of ZnnBuCl2- and LiZn2nBu2Cl4- were observed (Figures S5 and S6 of the Supporting Information). In addition, the complexes Zn2sBu2Cl3- and Zn2tBu2Cl3- were found, whose counterpart did not show considerable signal intensity for the case of LinBu/ZnCl2. The identities of ZnsBuCl2-, ZntBuCl2-,

figure

35

b

S1 S2 S3 S4

S7 S8 S9 S10 S12 S13 S14

S16 S19 S20 S21 S23 S25 S26 S27 S29

S31 S33 S35 Only one major

Figure 2. Relative signal intensities of zincate complexes produced by anion-mode ESI of 1-, 10-, and 50-mmolar solutions of LinBu/ ZnCl2 in thf.

and Zn2tBu2Cl3- were corroborated by CID (Table 1, entries 5-7, and Figures S7 and S9 of the Supporting Information). CID also helped to establish the composition of Zn2tBu2(OH)Cl2- (Table 1, entry 8, and Figure S10 of the



Supporting Information). The formation of Zn(OH)Cl2- upon fragmentation, eq 2, proves the identity of this unexpected complex.

Zn2tBu2(OH)Cl2- f Zn(OH)Cl2- + ZntBu2

(2)

Zn2tBu2(OH)Cl2- possibly originates from partial hydrolysis by traces of moisture. As one reviewer pointed out, an alternative genesis of Zn2tBu2(OH)Cl2- might involve ether cleavage of thf by the LitBu reagent. LitBu cleaves ethers much faster than LinBu,37 which would explain why no significant analogous hydroxo complexes were found for the LinBu/ZnCl2 solutions. To investigate this possibility further, we repeated the analysis of thf solutions of LitBu/ZnCl2, but this time prepared the solutions by addition of the organolithium reagent at -78 °C, followed by warming-up to 0 °C. This lower temperature is known to significantly reduce the rate of ether cleavage,37 whereas the transmetalation reaction does occur under these conditions.9 We still observed the formation of Zn2tBu2(OH)Cl2-, which suggests that it does not originate exclusively from ether cleavage. 3.1.3. LiCH3/ZnCl2. The preparation and analysis of methylzincate complexes proved to be more difficult than the investigation of other organozincates. Solutions of LiCH3/ZnCl2 in thf tended to form precipitates rather easily and yielded relatively low anion-mode ESI signal intensities. Apparently, methylzincate complexes exhibit a particularly high sensitivity toward residual water and/or oxygen contaminations. Nevertheless, the mass spectra obtained show the expected mononuclear Zn(CH3)Cl2-, together with ZnCl3-, as well as the polynuclear complexes Zn2(CH3)Cl4- and LiZn2(CH3)2Cl4- (Figure S11 of the Supporting Information). Zn(CH3)Cl2- produced Cl- as ionic fragment upon CID (Table 1, entry 9, and Figure S12 of the Supporting Information); Zn2(CH3)Cl4- and LiZn2(CH3)2Cl4predominantly yielded zincate species (Table 1, entries 10 and 11, and Figures S13 and S14 of the Supporting Information). 3.1.4. Li(C4H3S)/ZnCl2. Anion-mode ESI of a 1:1 mixture of zinc chloride and 2-lithiothiophene (Li(C4H3S)) in thf gave Zn(C4H3S)Cl2- as a base peak together with small amounts of ZnCl3- and complexes at higher m/z ratios, which were not investigated in more detail (Figure S15 of the Supporting Information). Like the other ZnRCl2- species studied, Zn(C4H3S)Cl2- formed Cl- and an organozinc chloride species upon CID (Table 1, entry 12, Figure S16 of the Supporting Information). 3.2. Solutions of 2 LiR/ZnCl2. 3.2.1. 2 LinBu/ZnCl2, 2 s Li Bu/ZnCl2, and 2 LitBu/ZnCl2. The anion-mode ESI mass spectra of 50-mmolar solutions of 2 LinBu/ZnCl2, 2 LisBu/ZnCl2, and 2 LitBu/ZnCl2 in thf all showed almost exclusively dibutylzincates ZnBu2Cl- (Figures 3, S17, and S18 of the Supporting Information). Upon CID, these complexes simply fragmented into Cl- and ZnBu2 (Table 1, entries 13-15, and Figures S19 and S21 of the Supporting Information). 3.2.2. 2 LiCH3/ZnCl2. Anion-mode ESI of a solution of 2 LiCH3/ZnCl2 in thf yielded the expected Zn(CH3)2Cl- complex (Figure S22), but only at rather low signal intensity. The higher signal intensity observed for Zn(CH3)Cl2- as well as the presence of significant amounts of ZnCl3- both indicate partial decomposition of Zn(CH3)2Cl- and related species. Like its butyl analogues, Zn(CH3)Cl2- fragmented into Cl- and diorganylzinc upon CID (Table 1, entry 16, and Figure S23 of the Supporting Information). Besides the mononuclear zincates mentioned, the (37) Gilman, H.; Haubein, A. H.; Hartzfeld, H. J. Org. Chem. 1954, 19, 1034.

Figure 3. Anion-mode ESI mass spectrum of a 50-mmolar solution of 2 LinBu/ZnCl2 in thf.

polynuclear species LiZn2(CH3)2Cl4- was detected as well. This complex was also present in LiCH3/ZnCl2 solutions in thf (see above). 3.2.3. 2 Li(C4H3S)/ZnCl2. The anion-mode ESI mass spectrum of a solution of 2 Li(C4H3S)/ZnCl2 in thf showed Zn(C4H3S)2Cl- as base peak (Figure S24 of the Supporting Information). As expected, CID of this complex produced Clas ionic fragment (Table 1, entry 17, and Figure S25 of the Supporting Information). Interestingly, in addition to Zn(C4H3S)2Cl-, small amounts both of Zn(C4H3S)Cl2- and Zn(C4H3S)3- were detected. The simultaneous presence of these two complexes points to a partial disproportionation of Zn(C4H3S)2Cl- according to eq 3. 2 Zn(C4H3S)2C1- h Zn(C4H3S)3 + Zn(C4H3S)C12 (3)

3.3. Solutions of 4 LiR/ZnCl2. 3.3.1. 4 LinBu/ZnCl2. Initial attempts to detect ZnnBu3- in 10-mmolar thf solutions of 3 LinBu/ZnCl2 prepared at room temperature did not prove successful. Instead of ZnnBu3-, mainly ZnnBu2Cl- was observed, and thus indicated an incomplete transmetalation. In order to bring the transmetalation to completion, we therefore employed 4 equiv. of LinBu and also increased the concentration to c(4 LinBu/ZnCl2) ) 50 mmol L-1 to minimize the potential problem of partial decomposition by traces of water and/or oxygen. Moreover, we prepared the thf solutions of 4 LinBu/ ZnCl2 at -78 °C and kept them at 0 °C, following the procedure reported by Uchiyama et al.9 The thus prepared solutions contained substantial amounts of analyte (20.5 mg/mL), which accumulated in the ESI source and usually caused complete clogging within 30 min or less. The anion-mode ESI mass spectrum of a 50-mmolar solution of 4 LinBu/ZnCl2 in thf indeed showed ZnnBu3- as base peak (Figure 4). Upon CID, this species lost up to two molecules of C4H8, thereby producing the mono- and dihydridozincates ZnHnBu2- and ZnH2nBu- (Table 1, entry 18, and Figure S26 of the Supporting Information). ZnHnBu2- also directly appeared in the anion-mode ESI mass spectrum of 4 LinBu/ZnCl2 in thf (Figure 4). Probably, the ZnHnBu2- observed here originated from unwanted fragmentation of ZnnBu3- during the ESI process despite the mild conditions applied. Alternatively, such a fragmentation reaction could have occurred already in solution. In addition, the anion-mode ESI mass spectrum showed small amounts of LiZnnBu4-. The low signal intensity did not permit a corroboration of this assignment by a CID experiment, but


Figure 4. Anion-mode ESI mass spectrum of a 50-mmolar solution of 4 LinBu/ZnCl2 in thf.

Figure 5. Anion-mode ESI mass spectrum of a 50-mmolar solution of LisBu/ZnCl2 in thf.

the exact m/z ratios observed point to the presence of a high number of hydrogen atoms (see above). Therefore, the assignment as LiZnnBu4- seems plausible. 3.3.2. 4 LisBu/ZnCl2. Anion-mode ESI of a 50-mmolar solution of 4 LisBu/ZnCl2 in thf yielded the expected trioganozincate ZnsBu3- (Figure 5). In perfect analogy to its ZnnBu3counterpart, this complex eliminated up to two molecules of C4H8 when subjected to CID (Table 1, entry 19, and Figure S27 of the Supporting Information). We also found evidence for the related LiZnsBu4- ion. While again, the only low signal intensity was not sufficient for a conclusive CID experiment, the isotopic pattern and the exact m/z ratios observed are compatible with this assignment (Figure S28 of the Supporting Information). In addition, the anion-mode ESI mass spectrum of the 4 LisBu/ZnCl2 solution showed a number of other species, most prominently a group of ions in the range 267 e m/z e 271. The measured isotopic pattern strongly suggests a mononuclear zincate, and the exact m/z ratios indicate the presence of a similar number of hydrogen atoms as in ZnsBu3-. Moreover, the ratio between the signal intensities of the ion at m/z ) 267 and its putative 13C isotopologue at m/z ) 268, I(267):I(268) ) 100: 13, is consistent with the presence of twelve carbon atoms. A structural assignment compatible with these findings is given by the butylperoxy zincate ZnsBu2(OOsBu)-. CID fully confirmed this assignment in that it produced the anions HOO-, (38) Schalley, C. A.; Schro¨der, D.; Schwarz, H.; Mo¨bus, K.; Boche, G. Chem. Ber./Recueil 1997, 130, 1085.


Figure 6. Anion-mode ESI mass spectrum of a 50-mmolar solution of 4 LiCH3/ZnCl2 in thf.

C4H7O-, and C3H5O2- (Table 1, entry 20, and Figure S29 of the Supporting Information), which are known to form in the fragmentation of the related tBuOO- anion38 and which would not be expected for a simple dioxygen adduct, i.e., (O2)ZnsBu3-. Other fragmentation processes observed produced the butylperoxide anion sBuOO- and hydridozincates (Table 1, entry 20). 3.3.3. 4 LitBu/ZnCl2. Solutions of 4 LitBu/ZnCl2 in thf much resembled their 4 LinBu/ZnCl2 and 4 LisBu/ZnCl2 counterparts and mainly produced ZntBu3- upon anion-mode ESI (Figure S30 of the Supporting Information). CID of this species again led to the elimination of up to two molecules of C4H8 and the formation of hydridozincates (Table 1, entry 21, and Figure S31 of the Supporting Information). The hydridozincate ZnHtBu2also directly appeared in the anion-mode ESI mass spectrum. In addition, ZntBu2Cl- was observed as well. 3.3.4. 4 LiCH3/ZnCl2. The detection of Zn(CH3)3- proved particularly challenging. Anion-mode ESI of 50-mmolar solutions of 4 LiCH3/ZnCl2 in thf gave rather low and unstable signal intensities. The main peak observed corresponds to Zn(CH3)2Cl(Figure 6), indicating incomplete transmetalation and/or partial decomposition of Zn(CH3)3-. Besides smaller amounts of Cland LiCl2-, the mass spectrum also shows peaks in the range 109 e m/z e 115. The ratio m/z ) 109 is the one expected for 64 Zn(CH3)3- but the experimental isotopic pattern is not consistent with that calculated for pure Zn(CH3)3-. A careful analysis revealed the presence of two different zincate species with partially overlapping isotopic patterns. We observed that the peak at m/z ) 109 was absent in the beginning of a measurement. The remaining peaks then displayed an isotopic pattern characteristic of (mononuclear) zinc (Figure S32 of the Supporting Information). CID of the ion with m/z ) 111 produced ionic fragments at m/z ) 17 and 95 (Table 1, entry 22, and Figure S33 of the Supporting Information). These dissociation processes can be rationalized by assuming the presence of Zn(CH3)2(OH)-, which can eliminate Zn(CH3)2 or CH4, eqs 4a and 4b, respectively.

Zn(CH3)2(OH)- f OH- + Zn(CH3)2

(4a)

Zn(CH3)2(OH)- f Zn(CH3)O- + CH4

(4b)

-

Zn(CH3)2(OH) presumably forms by partial hydrolysis. If any, the first part of the solution sampled should be prone to contamination by residual traces of moisture in the inlet system of the ESI source. The assignment as Zn(CH3)2(OH)- thus seems in line with the decline of its signal intensity during the measurement and the emergence of intact Zn(CH3)3-. The observed isotopic pattern in the range 109 e m/z e 115 can be


Figure 7. Detail of the anion-mode ESI mass spectrum of a 50mmolar solution of 4 LiCH3/ZnCl2 in thf (black) together with the expected isotopic patterns for Zn(CH3)3- (red) and Zn(CH3)2(OH)(blue) in a ratio of 5.0:1.0.


Figure 9. Anion-mode ESI mass spectrum of a 1-mmolar solution of 2 LinBu/ZnI2 in thf.

Figure 10. Cation-mode ESI mass spectrum of a 10-mmolar solution of LiCH3/ZnCl2 in thf. Figure 8. Anion-mode ESI mass spectrum of a 1-mmolar solution of 2 LinBu/ZnBr2 in thf.

fully explained by the simultaneous presence of Zn(CH3)3- and Zn(CH3)2(OH)-, as illustrated in Figure 7. The recorded isotopic pattern can be exactly reproduced assuming a 5:1 mixture of Zn(CH3)3- and Zn(CH3)2(OH)-. Unfortunately, though, the signal intensity of the former was too low for a CID experiment. 3.3.5. 4 Li(C4H3S)/ZnCl2. Anion-mode ESI of a 50-mmolar solution of 4 Li(C4H3S)/ZnCl2 in thf gave almost exclusively Zn(C4H3S)3- (Figure S34 of the Supporting Information). Upon CID, this species lost Zn(C4H3S)2 to produce C4H3S- (Table 1, entry 23, and Figure S35 of the Supporting Information). 3.4. Transmetalation of ZnBr2 and ZnI2. In order to assess the effect of the halide anion X- from the reactant zinc salt ZnX2, we also briefly studied the transmetalation reactions of ZnBr2 and ZnI2 with 2 equiv. of LinBu. Anion-mode ESI of the respective thf solutions produced the mononuclear organozincate complexes ZnnBuBr2- and ZnnBuI2- (Figures 8 and 9). Polynuclear organozincates were absent, whereas a manifold of lithium halide clusters LinXn+1- were observed. Diorganozincate complexes ZnnBu2X- were not detected either, which might be due to the rather low concentrations of the organometallic species (c(LinBu) ) 2 mmol L-1) and their partial decomposition by traces of moisture/and or oxygen. 3.5. Cation-Mode ESI Mass Spectra. The cation-mode ESI mass spectra of thf solutions of various lithium organozincate complexes uniformly yielded Li(thf)3+ in high signal intensities (Figure 10). Besides, smaller amounts of Li(thf)2+, Li(thf)4+, and Li2Cl(thf)4+, but no zinc-containing complexes were detected.

Table 2. Relative Rate Constants and Branching Ratios of the Ion-molecule Reactions between Organozincate Ions and Formic Acid reactant ion

krela

product channel

branching ratio

ZnnBu2ClZnsBu2ClZnnBu3-

10 ( 5

Aggregation and Reactivity of Organozincate Anions Probed by

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